For fifty years B?rbel Inhelder (1913-1997) was the research companion of Jean Piaget. In this unique volume, published in her honour, leading international researchers examine the various aspects of her work, her ideas and her contribution to developmental psychology. While the essays explore Inhelder's work with her more famous colleague, they also highlight areas of research in which her ideas were at variance with those of Piaget, such as mental imagery, and areas in which her innovations have not been fully recognised, such as her discovery of the formal operations stage - an event usually attributed to Piaget - and her introduction of longitudinal studies in the field of cognitive development. The final chapter, written by Inhelder herself, deals with experimental reasoning in children and adolescents and provides a glimpse of her creativity.